The bearing for supporting a driving motor used in the EV and HEV is required to be operable within a wide temperature range because the bearing is used under various conditions, e.g., in low temperature atmospheres of cold regions and under the elevated temperature conditions due to running of the motor, transmission or reducer. In line with the rotation of bearing at higher speed resulting from higher output of the motor, the bearing is also required to have durability against high-speed rotation in order to improve the rotational performance.
In light of the operating conditions of high temperatures, the grease used for the bearings is required to have a long seizure life. The greases using mineral oil as a base oil and those using lithium soap as a thickener cannot meet the requirement of the seizure life under the high-temperature circumstances because of insufficient heat resistance of the base oil and the thickener.
To improve the seizure life under the high-temperature conditions, for example, JP 1-259097 A proposes a grease composition comprising an alkyldiphenyl ether oil as the essential ingredient, and a particular thickener. However, the above-mentioned grease composition cannot meet the low-temperature flowability because of high kinematic viscosity of the base oil.
For the purpose of satisfying the low-temperature flowability of the grease, it has been regarded as the usual measures to lower the kinematic viscosity of the employed base oil. For example, JP 2000-198993 A proposes a grease composition where a base oil contains an ester oil with a kinematic viscosity at 40° C. of 10 mm2/s or more. When the kinematic viscosity of the base oil is lowered, the low-temperature flowability can be satisfied, but the base oil exhibits poor heat resistance under the high temperature conditions, whereby satisfactory seizure life cannot be ensured.